US2713133A - Germanium diode and method for the fabrication thereof - Google Patents

Description

United States Patent *O GERMANIUM DIODE AND METHOD FOR THE FABRICATION THEREOF Peter L. Ostapkovich, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania i Application June 5, 1952, Serial No. 291358 13 Claims. (Ci. 317-239) The present invention relates to improvements in the composition and manufacture of asymmetrically-conductive semi-conductor devices, and more particularly it relates to improved crystal rectifiers suitable for use in frequency Conversion.

There are many well known applications of crystal rectifiers in which the noise performance thereof is oi utmost importance. For example, such crystal rectiers are commonly used as mixers for converting radio frequencies to intermediate frequency signals in high gain superheterodyne receivers, such as may be employed in long-range communication ssytems or radar systems. In such receivers, the eectiveness of increasing the gain of amplifiers following the crystal mixer is limited principally by the noise generated in the crystal. When the noise generated in the crystal is substantially greater than the signal to be received, increases in receiver gain are no longer effective to improve the useful sensitivity. The amount of noise generated by the crystal mixer will, therefore, generally place an upper limit upon the maximum separation of transmitter and receiver in a communication system, or upon the maximum range of a radar system for example.

One generally-accepted criterion of noise performance for a crystal in such applications, is the noise figure F, defined as the product of the conversion loss and the noise temperature of the crystal. As has been indicated hereinbefore, the crystal mixer is usually by far the greatest contributor to the overall noise figure of the receiver, and is therefore at least an approximate index to the maximum signal sensitivity of which the receiver is capable. For these and other reasons, it has become highly desirable to produce crystals suitable for mixers and detectors which are characterized by the lowest possible noise figure.

In the past, mass production of crystals having con- .sistently low noise figures has not been possible. The procedure generally utilized heretofore has been to produce the crystals in large numbers, and then to select, by individual testing, the relatively small fraction thereof which are characterized by adequately low noise figures. The noise figures of even these selected crystals have typically been of the order of 10 to 15 decibels, for example. Although occasionally a crystal of substantially lower noise figure might be obtained by these methods, such anomalous improvements in noise figure were not understood, and could not be obtained consistently. It is obvious that such a process in which each crystal must be tested individually and a large number rejected, is ineflicient and expensive.

Another related difiiculty which has been experienced in obtaining low-noise crystals, arises from the fact that it is customary to cut the final crystals from a relatively large ingot which is usually not entirely homogenous. Typically, the noise figures of the cut crystals have been quite critically dependent upon the position in the ingot from which they came, only a relatively small portion of the ingot producing crystals of accept- 'ice able noise figures. The convenient method of growing large crystal ingots and cutting the final small crystals therefrom, has therefore been wasteful and inefiicient when low-noise crystals were required.

It is therefore an object of my invention to provide asymmetric semiconductve devices of substantially improved noise figures.

Another object is to provide such devices which may be produced in large quantities, While maintaining the noise figures thereof consistently low.

Still another object is to provide a composition of matter suitable for use in the rectification of high frequency signals, which provides consistently low noise figures.

A further object of the invention is to provide a relatively large body of semiconductve material of such nature that small crystals, cut from widely-separated portions of the body, will be characterized by substantially uniform noise figures.

Still another object is to provide an improved method for the fabricaton of low-noise semiconductor devices.

In accordance with my invention, the above objectives are attained by constituting the crystalline semiconductive body of the rectifying element from a new and unexpectedly advantageous composition of matter. Whereas formerly it had been believed that best results with regard to noise figure would be obtained by employing a semiconductive material comprising germanium in the purest form obtainable plus only a principal or primary impu'ity, I have found that substantially improved noise gures are obtained from crystals containing, in addition, a secondary impurity in an amount which is an order of magnitude less than that of the primary impurity.

More specifically, the preferred embodment of my invention employs a composition of matter which is composed of bismuth in an amount between substantially 0.0l% and 004% by weight, antimony in an amount between substantially 0.1% and 03% by weight, and the remainder of germanium which is substantially free from other significant impurities. It will be understood hereinafter that the term significant impurities refers to materials which act as donors or acceptors of electrons in the germanium crystal lattice. while the term insignicant impurities refers to those irnpurities which do not act to any substantial extent as either donors or acceptors.

I have found that when the amount of the secondary impurity bismuth is substantially less than 0.0l% or substantially greater than 0.04% by weight, noise figures comparable to those of the prior art are obtainedr However, When the bismuth content lies within a critical range extending from substantially 001% to (104%, very substantial reductions in noise figure are obtained. Preferably, the amount of bismuth is near the center of this range at approximately 002%, for which value noise figures of about 6 to 8 db are consistently obtained, with occasional instances of noise figures of 5 db or less.

The noise figures of the crystals thus produced are relatively uncritical with respect to variations of the bismuth content about the preterred value of 002% and within the specified range. Thus, the bismuth 'content may be varied from 0.015 to 0.03% Without seriously increasing the noise figures obtained, This lack of crit-- icalness as to bismuth content within the specified range is believed to be responsible at least in part for the fact that consistently low noise figures can be obtained. for crystals derived from widely-separated portionsof' concentration are apparently not su'hciently great to produce any" substantial non-uniformitiesin the noise figures of crystals derived from difierent portions of an ingot of the normal practical dimensions. Thus I have found'that, for aningot having a length of three centimeters', the noise figures obtained have substantially the same low values substantially r egardless of 'the positions within the surface of' the ingot from which the cut crystals, are derived.

Other advantages and features of the invention will t become apparent from a consideration of the following detailed description taken in connectionwith. the accompanying drawings, in which:

Figure l is a sectional View of a typical Crystal cartridge containinga semiconductive materialin accordance with the inventiom, and

Figure 2 is a graph illustrating the eliects of various amounts of bismuth upon the noise figure of crystals constituted' in accordance with the invention.

In Figure 1, there is shown a Crystal cartridge assembly of the coaxial type embodying the invention, and suitable for use as a rectifier or mixer in the conversion of radio-frequency signals of approximately 10,000 mega- 'cycles per second, to intermediate frequency signals of approximately 60 megacycles per second, for example. Thisassembly may be generally conventional except for the constitution of the Crystal material itself. Thus, the assembly may comprise a ceramic Cartridge case 1 in i the form of a hollow cylinder, threaded at each end, and

having' a circular opening 2 through one wall' thereof. Into one end of Cartridge 1 there may be inserted threadedplug member 3, which is hollow so as to accommodate a crystal-bearing rod member 4 press-fitted into it; The crystal member 5' may be soldered to rod 4, and is point-contacted by means of a whisker 6 of, a suitable material such as' tungsten or titanium, welded or^ otherwise fastened to a Whisker-bearing plug member 7 threaded into the opposte end of Cartridge 1. If' desired, the Crystal assembly may beimpregnated with a, suitable waX material, and opening 2 may be sealed with.

an appropriate sealing wax.

The Crystal member 5 is preferably composed princpally of nearly pure germanium, to the extent of 99.78%

by weight in the preferred embodiment, alloyed with antimony intthe amount of 02% by Weight, and bismuth in the amount'of 002% by weight. Although the. germanium content is preferably substantially pure, the 99.78% of germanium may also include an appreciable amount of insignificant impurities, such as sihcon and carbon, for example. these insignificant impurities may be as great as 078% by weight, for example. However, With regard to, significant impurities, other than the antimony and bismuth in the amounts specified, the amounts of such impurities should be maintained below 001% and preferably on the order of .001% or less, by weight.

The effects of the addition of bismuth are represented. generally in Figure 2. It will be understood with regard to this'figu'rethat the'exact nature of the variation of noise figure with bismuth content isnot precisely known for all values of bismuth content, but that sufficent information has been obtained to indicate that this variation possesses the general characterstics` shown by the graph.

Referring to Figure 2 in more detail, the ordinates of the graph represent the noise figures of antimony-doped germanium crystals expressed in decibels, while the, ab

These were-used as mixers in a superheterodyne receiver of' In some instances, the amounts of microwaves in the 10,000 megacycle band. However, it i will be understood that the advantages of the invention are not limited to use, at these frequencies,. experiments having shown that similar improvements in noise obtained.

4, figure are obtained at 1,000 megacycles per second, for example; For-bismuth percentages* of' less than-'0.01' percent, the noise figure is substantially equal to 13 decibels, which is typical of prior art Crystal& However, above 0.01 percent the noise figure dips sharply to a value of about 6 decibels, and then, risesv again to substantially 13 decibels for bismuth contents of about 0.04 percent. Throughout the entire region from 0.01 percent to 0,04 percent of bismuth' by weight, improvements in noise figure with respect to the prior' art are obtained, and within the substantial range between 0.0l5, percent and" 0.03 percent the noise figure is substantially equal to 6 decibels. As noted hereinbefore, it is believed to be this uniformly excellent noise performance obtained over a substantial range of bismuth contents, within the prior art, with the important exception of'the addition: of

the secondary impurity bismuth along with the primary impurity antimony; Thus, the process of produci'ngthe Crystal material normally begins with, germanium oxide which is reduced to provide relatively pure germnium.

This ge'manium may then be further purified by melting it, progressively cooling it so as'to concentrate impurties in one end, and thenselecting the puified' POOITOf the germanium for subsequent use. This puriedgermanium may then be placed into a crucible arranged for vertical ingot production by placement within aradint" heating device suitably arranged to provide an appropriate temperature gradient through the germanium material. The germanium is melted by means of heat from the above-mentioned radiant heaters, and then` caused to.: solidify progressively and` slowly under carefully' controlled conditions so as to obtain a relatively'large ingot` of germanium which is single crystalline. However, before cooling and solidification, and preferably before melting, the primary and secondary' impurities, constituting antimony and bismuth in the. amounts hereinbefore' indicated, are added to the germanium. The singlecrystal ingot produced therefore comprises an alloy of germanium, antimony and bismuth, in aecordance with-the' percentages indicated hereinbefore. This' ingot may be generally cylindrical in form, and may commonly havea' length of 2 to.3 tcentimeters, for example.

The Crystal ingot may then be cut into appropriate"` small'tslabs by. means of a diamond cutting wheel, soldered to the rod member 4of- Figure l, and provided With asuitable high polish.

The whisker 6, appropriately cut, pointed andcrmped, r

may bewelded or soldered to threaded plug member 7: Asuitable'cement may then be applied to thethreads of' plug member, 7, after which it isscrewed into one end 'of the whisker and then advanced another two thousand'ths of an inch. A D.-C. forming: current of 0.1 amperemay be passed through the contact between whisker'and- Crystal, until the desired, voltage-current characteristic is ,proper amounts, it is believed unneccssary; to provide hereinia: further detaileds description thereof;

Although the invention has been' describedtwith spe-,l i

If desired, suitable wax impregnation: may" n n 444 AA. A

cific reference to certain applications thereof, it will be app: ciated that it is actually susceptible of embodiment in any of a wide variety of forms without departing from the spirit or" the invention. in particular, the invention is obviously not limited to any specific form of geometry or Construction of the Crystal Cartridge, or of any of the parts thereof with the exception of the Constitution of the crystal plate itself, or to any particular apparatus for practicing the disclosed method of fabrication.

I claim:

i. As a semiconductve material for asymmetricallyconductve devices, that composition of matter which comprises substantially 0.01 percent to 0.04 percent of bismuth by weight and 0.1 percent to 0.3 percent of antimony by weight, the remainder comprising germanium substantially free from other significant impurities.

2. The composition of claim 1, in which the bismuth content is substantially equal to 0.02 percent by weight.

3. The composition of claim 2 in which the antimony content is substantially equal to 0.2 percent by weight.

4. The composition of claim 3, in which said remainder is free from significant impurities having concentrations greater than 0.01 percent by weight.

5. A germanium crystal of improved electrical characteristics, said erystal consisting essentially of germanium, with a primary impurity of antimony in an amount between the limits of substantially 0.1 percent and 0.3 percent by weight, and with a secondary impurity of bismuth in an amount between the limits of substantially 0.01 percent and 0.04 percent by weight.

6. The crystal of claim 5, in which the bismuth content is substantially equal to 0.02 percent by weight.

7. The crystal of claim 5, in which the antimony content is substantially equal to 0.2 percent by weight.

8. A germanum crystal rectifier of improved noise performance, said rectifier comprising: a body of semiconductive material comprising bismuth in an amount between substantially 0.01 percent and 0.04 percent by weight and antimony in an amount between substantially 0.1 percent and 0.3 percent by weight, the remainder compn'sing germanium substantially free from other significant impurities; and a metallic Contacting element making small-area contact with said body of material.

9. The rectifier of claim 8, in which the bismuth coi tent is substantially equal to 0.02 percent by weight.

10. The method of fabricating an ingot of semiconductive material for use in signal translating devices, said method comprsing the steps of: forming a melt consisting essentially of germanium, antimony in an amount between substantally 0.1 percent and 0.3 percent by weight, and bismuth in an amount between substantially 0.01 percent and 0.04 percent by weight; and cooling said melt to produce a crystalline semiconductive ingot.

ll. The method of fabricating an ingot of semiconductive material for use in signal translating devices, said method comprising the steps of: forming a melt consisting essentially of germanium, antimony in an amount between substantially 0.1 percent and 0.3 percent by weight, and bismuth in an amount substantially equal to 0.02 percent by weight; and cooling said melt to produce a crystalline semiconductive ingot.

12. The method of manufacturing semiconductive crystal bodies of improved noise figures, said method comprising the steps of: forming a melt consisting essentially of germanium, antimony in the amount of substantially 0.1 percent to 0.3 percent by weight, and bismuth in the amount of substantially 0.01 percent to 0.04 percent by weight; cooling said melt to produce a crystalline ingot; and dividing said ingot into a plurality of smaller crystal bodies.

13. The method of manufactuing semiconductive crystal bodies of improved noise figures, said method comprising the steps of formiug a melt consisting essentially of germanium, antimony in the amount of substantially 0.1 percent to 0.3 percent by weight, and bismuth in an amount substantially equal to 0.02 percent by weight; cooling said melt to produce a crystalline ingot; and dividing said ingot into a plurality of smaller Crystal bodies.

References cited in the file of this patent UNITED STATES PATENTS 2,514,879 Lark-Horovitz et al July 11 1950 2,583,008 Olsen Jan. 22, 1952 2,602,211 Sca et al July 8, 1952

Claims (1)

1. AS A SEMICONDUCTIVE MATERIAL FOR ASYMMETRICALLYCONDUCTIVE DEVICES, THAT COMPOSITION OF MATTER WHICH COMPRISES SUBSTANTIALLY 0.01 PERCENT TO 0.04 PERCENT OF BISMUTH BY WEIGHT AND 0.1 PERCENT TO 0.3 PERCENT OF ANTI-

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